1. Field of the Invention
Methods and apparatuses consistent with the present invention relate to a wireless local area network (LAN), and more particularly, to carrier sensing multiple access with collision avoidance (CSMA/CA) in a wireless LAN in which high throughput (HT) stations and IEEE 802.11 legacy systems coexist.
2. Description of the Related Art
Medium access control (MAC) using a carrier sensing multiple access with collision avoidance (CSMA/CA) method is used in a wireless LAN.
In the CSMA/CA method, physical carrier sensing and virtual carrier sensing are used for carrier sensing. In physical carrier sensing, a physical layer (PHY) determines whether a received power equal to or greater than a specific value is detected, and informs a medium access control (MAC) layer of whether a medium is in a busy or idle state, thereby sensing a carrier. In virtual carrier sensing, if an MAC protocol data unit (MPDU) can be correctly extracted from a received PHY protocol data unit (PPDU), a header field of the MPDU, that is, a duration/identification (ID) field, is analyzed, and the medium is deemed to be in the busy state during a scheduled time for using the medium. Stations determine whether the medium is in the busy state by using the two carrier sensing methods, and do not access the medium if it is in the busy state.
Referring to FIG. 1A, an MAC header of a frame transmitted through a general IEEE 802.11 wireless LAN includes duration information indicating a time between when the frame is transmitted and when an acknowledgement (ACK) frame is received to confirm that the frame is received. After receiving the frame, the stations analyze the MAC header so that medium access is not tried for a certain duration of time, thereby avoiding collision. According to a feature of a wireless medium, all stations connected through the wireless LAN can physically receive all frames transmitted in a radio wave coverage area even if the frame is sent to a particular station.
As described above, in virtual carrier sensing, the CSMA/CA can be effectively used only when the MAC protocol data unit/PHY service data unit (MPDU/PSDU) are analyzed without errors. That is, virtual carrier sensing can only be carried out when an MAC header value of a data frame of a station waiting for medium access can be correctly read.
However, if a transmitting station sends data with a high data transfer rate (Tx Rate) and an error occurs due to an unstable channel condition, or a receiving station cannot cope with the high data transfer speed, virtual carrier sensing cannot be carried out because the received MPDU/PSDU cannot be analyzed. Therefore, the CSMA/CA method becomes ineffective. Accordingly, when a legacy station operating in accordance with the IEEE 802.11a, IEEE 802.11b, and/or IEEE 802.11g standards and a high throughput (HT) station having a higher capability than that of the legacy station (for example, an multi-input-multi-output (MIMO) station or a station using a channel bonding scheme) coexist in the wireless LAN, and when an HT format is sent, the legacy station cannot analyze the HT format frame, and thus virtual carrier sensing cannot be correctly carried out.
FIG. 1B illustrates a configuration of a data frame used in the IEEE 802.11a standard. Referring to FIG. 1B, duration information can be predicted by analyzing information regarding the RATE field value and LENGTH field value included in the signal field of the PHY header, thereby avoiding medium access collision.
At present, the IEEE 802.11n standard is in development. Referring to FIG. 2, in the IEEE 802.11n standard, if HT stations and legacy stations coexist in the wireless LAN, a legacy format PHY header (L-Preamble, L-SIG) is added into an HT format data frame so as to be recognized by the legacy stations. In addition, a time required after an L-SIG field begins and until an ACK frame is received is recorded in the L-SIG field.
In other words, since the legacy stations can recognize the RATE field and the LENGTH field included in the legacy format PHY header when the legacy stations receive an HT format frame, even if the legacy stations cannot analyze the HT format MAC header, the legacy stations can determine that the medium is being used during that duration of time, thereby avoiding collision. Hereinafter, the time specified by the RATE field and the LENGTH field in the legacy format L-SIG field added into the HT format data frame will be referred to as an L-SIG Tx Opportunity Protection Period (LTPP).
When the LTPP mechanism is used, medium access collision can be avoided, but stations have to contend unfairly to attain permission for medium access. This will be described in reference with FIG. 3.
Referring to FIG. 3, when the LTPP mechanism is used, even if a legacy station can read the PHY header, the legacy station cannot read the next fields, that is, HT format, which leads to an error. Then, the PHY, or baseband layer, indicates the error occurrence to the MAC layer.
The error indication begins when the LTPP ends. From this point, the legacy station participates in contention for medium access after waiting for a longer time than the HT station. This is because, when an error occurs because the legacy station cannot read the HT format frame, the legacy station starts to back-off after standing by for a time defined in extended inter-frame space (EIFS, 94 us in IEEE 802.11a). This is different from the HT station which starts to back-off after standing by for a time corresponding to DCF inter-frame space (DIFS, 34 us in IEEE 802.11a). Accordingly, the legacy station becomes disadvantageous to other HT stations in terms of contending for medium access.